A piston reciprocates within a cylinder of an internal combustion engine and compresses fluids, such as gases, within a combustion chamber of the cylinder. These compressed fluids are then ignited to expand within the combustion chamber thereby forcing the piston away from the point of ignition and cycling the piston to its original position. Pistons typically include at least one groove for receiving a piston ring. The piston ring forms a seal with the wall of the cylinder to prevent gases from escaping from the combustion chamber.
There are traditionally two different types of piston rings, oil control rings and compression rings. Regarding compression rings, a piston assembly typically includes one or more compression rings to generate a seal between the outer surface of the piston and the wall of the cylinder. An inner peripheral face of the ring fits into the ring groove of the piston while a portion of an outer peripheral face of the compression ring contacts the wall of the cylinder. The outer peripheral face of the compression ring generates a seal in the gap between the piston and the cylinder wall to prevent high-pressure combustion gases and air from escaping the combustion chamber.
A particularly effective compression ring well-known in the art is a Napier-styled ring 10, as seen in FIG. 1. The Napier-style ring 10 includes a generally tapered outer peripheral face 12 and a lower surface 13 having a hook groove 14. The intersection between the hook groove 14 and the tapered outer peripheral face 12 define an edge 16 that contacts the wall 18 of the cylinder 20 when the ring 10 is positioned within a groove 22 of a piston 24.
To improve the seal generated by traditional piston rings 10, manufacturers have found it desirable to twist the rings 10 within the grooves 22 of the piston 24. Twisting the ring 10 results in the edge 16 of the outer peripheral face 12 of the piston ring 10 bearing against the wall 18 of the cylinder 20 with an increased force as compared to the rest of the outer peripheral face 12. This increased force on the edge 16 generates a more effective seal and prevents leakage of gases, air and lubricating oils between the cylinder wall 18 and the outer peripheral face 12 of the piston ring 10. Furthermore, twisting the ring 10 within the groove 22 eliminates passageways 26 between surfaces of the piston ring 10 and boundaries of groove 22 to provide continuity of the seal and prevent the escape of gases from the combustion chamber through the passageways 26. While manufacturers continually search for ways to increase the amount of twist of the piston ring 10 within the groove 22, a natural, inherent twist typically exists. The natural twisting results from the cycle of the piston 24 within the cylinder 20. The cycle of the piston 24, along with the contact of the edge 16 of the outer peripheral face 12 with the wall 18 of the cylinder 20, produces a minimal amount of natural twisting of the piston ring 10 within the ring groove 22.
Twisting the compression ring 10 within the groove 22 of the piston 24 is also beneficial for Napier-style compression rings 10. The edge 16 defined by the hook groove 14 will contact the wall 18 of the cylinder 20 with increased force as compared to other portions of the tapered outer peripheral face 12 to provide an effective seal. Further, elimination of the passageways between surfaces of the piston ring 10 and boundaries of the groove 22 is beneficial to prevent escape of combustion gases. However, twisting Napier-styled rings 10 has yielded other challenges for piston ring manufacturers. Specifically, twisting of Napier-styled rings 10 has increased the occurrence of a phenomenon commonly known as ring collapse.
Ideally, the edge 16 of the outer peripheral face 12 is in contact with the wall 18 of the cylinder 20 to prevent gases from escaping. However, in some instances, combustion gases enter a gap 30 disposed between the outer peripheral face 12 and the wall 18 of the cylinder 20. Forces generated by the combustion gases press down-ward upon the outer peripheral face 12 to separate the edge 16 from the wall 18. This separation between the edge 16 and the wall 18 permits combustion gases to escape from the combustion chamber and is commonly known as ring collapse. As a result, ring collapse reduces the effectiveness of piston rings 10.
Ring collapse is particularly prevalent with Napier-style rings 10 because of the tapered outer peripheral face 12. The taper generates a gap 30 typically larger in area than the gap 30 associated with traditional piston rings. Accordingly, a greater portion of the tapered outer peripheral face 12 is exposed to the combustion gases and more force presses downward to separate the edge 16 from the wall 18 of the cylinder 20 than with traditional piston rings. Further, although twisting the ring 10 within the groove 22 has some beneficial results, twisting also produces a larger gap 30 than gaps 30 associated with traditional, non-twisted rings.
To compensate for the detrimental effects of the ring collapse phenomenon, manufacturers attempt to eliminate or at least minimize twisting of the piston ring 10 within the ring groove 22, even the natural twisting that occurs during the cycle of the piston 24 within the cylinder 20. To minimize the natural twist, manufacturers typically add a bevel 28 between the inner peripheral face 26 and the lower surface 13, as seen in FIG. 2. However, producing the bevel 28 requires a time-consuming and costly additional machining process, thereby increasing the overall cost of the piston ring 10.
Accordingly, there is a need for an improved piston ring that provides an effective seal between an outer peripheral face of the ring and a wall of a cylinder, utilizes the positive effects of twisting the piston ring within the ring groove, but reduces the occurrence of ring collapse without additional machining processes.